The term "thick film resistor" as used in the electronics industry refers to the method employed to fabricate a resistor rather than the relative thickness of the individual layers of material which comprise the resistor. In the manufacture of a thick film resistor, especially formulated conductor and resistor pastes are applied to and fired on a substrate in a predetermined sequence. In the manufacture of conventional prior art thick film resistors, a conductor paste, which typically contains a precious metal such as gold, is printed on a substrate in a predetermined pattern, dried and then fired to form a pair of spaced apart metal terminals. Thereafter, a layer of a resistor paste, which is typically comprised of a mixture of a dielectric glass frit and electrically conductive particles, is printed on the substrate between and in electrical contact with the terminals. The resistor paste is dried and then fired which causes the glass frit to fuse, forming a resistor body having a glassy matrix with conductive particles distributed throughout the matrix. The resistance of the resistor which is obtained is determined to a large extent by the relative amount of conductive particles in the resistor body.
In certain applications, such as in high power microwave transmission, it is often necessary that the resistors be capable of handling relatively large amounts of power. To provide such resistors, it is common practice to increase the surface area of the resistor by widening or lengthening the resistor body. This permits greater power dissipation and maintains the operating temperature of the resistor below the point where destructive, irreversible changes occur. However, increasing the surface area of thick film resistors is not a satisfactory solution in microelectronic applications because they occupy excessively large surface areas of the devices.
In order to overcome the inherent problems of conventional thick film resistors, a novel type of resistor structure was suggested by Landry et al. in U.S. Pat. No. 4,245,210 entitled "Thick Film Resistor Element And Method Of Fabrication," the disclosure of which is hereby incorporated by reference. The Landry et al. resistor is comprised of a substrate, a pair of terminals and a series of from, for example, at least three and up to ten or even more overlying layers of a high resistivity material. The use of multiple layers of high resistivity materials, as taught by Landry et al., has made it possible to form low resistance resistors with improved power handling and high voltage capability.
The Landry et al. resistors, however, do have certain shortcomings. The fabrication of the Landry et al. resistors require multiple printing, drying and firing steps in order to deposit a sufficient number of layers of high resistivity material. The relatively large number of firing steps, in addition to increasing the production cost of the Landry et al. resistors, also tends to cause the conductive particles in the resistor composition of each of the previously fired layers to migrate towards the upper surface of the resistor structure. The resulting migration of the conductive particles causes the final resistor structure to have relatively low resistance adjacent the upper surface and relatively high resistance adjacent the substrate and terminals. This results in a substantial reduction of the power handling capability of the multiple layered resistors taught by Landry et al.
What would be highly desirable would be a resistor smaller in area than a conventional thick film resistor which is simpler to fabricate than the Landry et al. resistor and which has improved power handling capabilities.